Three phase inverter system using an eight-switch-three-phase unfolder

ABSTRACT

A three phase inverter can include: a first converter and a second converter connected to an input source in parallel, respectively; a first single phase inverter connected to the first converter through a first inverter first input node and a first inverter second input node and providing a first inverter first output node and a first inverter second output node; a second single phase inverter connected to the second converter through a second inverter first input node and a second inverter second input node and providing a second inverter first output node and a second inverter second output node; and a common output node connected to the first inverter first output node and the second inverter first output node.

BACKGROUND

Compact three phase inverters have a significant importance in theintegration of high power embedded renewable energy sources, nanogrids,and electrification of transportation, where the size and weight of theDC/AC power processing system are critical design aspects. However,these three phase inverters require a large number of high frequencyswitches, which are expensive and require a bulky cooling system.

BRIEF SUMMARY

Embodiments of the subject invention provide novel and advantageousthree phase inverters that use an eight-switch-three-phase unfoldercircuit operating at a line frequency, thereby reducing the number ofhigh frequency switches, thus minimizing the cooling system requirement.

In an embodiment, a three phase inverter can comprise: a convertercircuit providing a first output voltage and a second output voltage; afirst unfolder including a first leg and a second leg that are connectedto the first output voltage through a first unfolder first input nodeand a first unfolder second input node; and a second unfolder includinga third leg and a fourth leg that are connected to the second outputvoltage through a second unfolder first input node and a second unfoldersecond input node, a first unfolder first output node of the first legbeing connected to a second unfolder first output node of the third legthrough an unfolder common output node, the second leg providing a firstunfolder second output node, the fourth leg providing a second unfoldersecond output node, each of the first to fourth leg comprising twoswitches.

In another embodiment, a three phase inverter can comprise: a firstconverter and a second converter connected to an input source inparallel, respectively; a first single phase inverter connected to thefirst converter through a first inverter first input node and a firstinverter second input node and providing a first inverter first outputnode and a first inverter second output node; a second single phaseinverter connected to the second converter through a second inverterfirst input node and a second inverter second input node and providing asecond inverter first output node and a second inverter second outputnode; and a common output node connected to the first inverter firstoutput node and the second inverter first output node.

In yet another embodiment, a three phase inverter can comprise: a firstDC-DC converter connected to an input source; a second DC-DC converterconnected to the input source; a first inverter first switch connectedto the first DC-DC converter through a first inverter first input node;a first inverter second switch connected to the first DC-DC converterthrough a first inverter second input node and connected to the firstinverter first switch through a first inverter first output node; afirst inverter third switch connected to the first DC-DC converterthrough the first inverter first input node; a first inverter fourthswitch connected to the first DC-DC converter through the first invertersecond input node and connected to the first inverter third switchthrough a first inverter second output node; a second inverter firstswitch connected to the second DC-DC converter through a second inverterfirst input node; a second inverter second switch connected to thesecond DC-DC converter through a second inverter second input node andconnected to the second inverter first switch through a second inverterfirst output node; a second inverter third switch connected to thesecond DC-DC converter through the second inverter first input node; asecond inverter fourth switch connected to the second DC-DC converterthrough the second inverter second input node and connected to thesecond inverter third switch through a second inverter second outputnode; a common output node connected to the first inverter first outputnode and the second inverter first output node; and a controllerproviding a first signal to the first inverter second switch and thefirst inverter third switch, a second signal to the first inverter firstswitch and the first inverter fourth switch, a third signal to thesecond inverter first switch and the second inverter fourth switch, anda fourth signal to the second inverter second switch and the secondinverter third switch.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1(a) shows a three phase voltage source inverter for Buck operationonly.

FIG. 1(b) shows a three phase voltage source inverter with aunidirectional boost converter.

FIG. 1(c) shows a three phase voltage source inverter with abidirectional boost converter.

FIG. 2(a) shows a unidirectional three phase Z source inverter.

FIG. 2(b) shows a bidirectional three phase Z source inverter.

FIG. 2(c) shows a unidirectional three phase quasi Z source inverter.

FIG. 2(d) shows a bidirectional three phase quasi Z source inverter.

FIG. 3(a) shows a unidirectional three phase split source inverter.

FIG. 3(b) shows a bidirectional three phase split source inverter.

FIG. 4(a) shows a generic three phase differential mode inverterconnection diagram.

FIG. 4(b) shows a non-isolated Ćuk three phase differential modeinverter.

FIG. 4(c) shows an isolated Ćuk three phase differential mode inverter.

FIG. 5(a) shows a three phase inverter system using an unfolder circuitaccording to an embodiment of the subject invention.

FIG. 5(b) shows a controller of a three phase inverter system accordingto an embodiment of the subject invention.

FIG. 5(c) shows waveforms of a three phase inverter system according toan embodiment of the subject invention.

FIG. 6 shows key waveforms of a three phase inverter system according toan embodiment of the subject invention.

FIG. 7 shows an isolated unidirectional version of a three phaseinverter system for a photovoltaic interface application according to anembodiment of the subject invention.

FIG. 8 shows a non-isolated bidirectional version of a three phaseinverter system for energy storage interface application according to anembodiment of the subject invention.

FIG. 9 shows an isolated bidirectional version of a three phase invertersystem for energy storage interface application according to anembodiment of the subject invention.

DETAILED DESCRIPTION

Embodiments of the subject invention provide novel and advantageousthree phase inverters that use an eight-switch-three-phase (ESTP)unfolder circuit operating at a line frequency, thereby eliminating a DClink capacitor and minimizing the cooling system requirement.

A three phase inverter system of an embodiment of the subject inventionreduces the number of high frequency switching semiconductor devices inthe three phase inverter system, which reduces the total cost, weightand size. Three phase inverter systems of embodiments of the subjectinvention can significantly reduce the number of high frequencyswitching semiconductor devices, thereby can be effectively used in theindustries including the renewable energy systems and electric vehicles.

FIGS. 1(a), 1(b), and 1(c) show a conventional three phase voltagesource inverter (VSI). FIG. 1(a) shows a three phase VSI for Buckoperation only, which is the most commonly utilized three-phaseinverter. Referring to FIG. 1(a), the VSI comprises three legs inpush-pull configuration, and a common-mode voltage has to be added tothe output of each leg. In this three-phase buck VSI, six switchesoperating at high frequency are required.

FIGS. 1(b) and 1(c) show a three phase voltage source inverter with aunidirectional boost converter and with a bidirectional boost converter,respectively. Referring to FIGS. 1(b) and 1(c), a boost stage can beadded before the three-phase buck VSI to increase the voltage level ofthe dc supply (Vin), and this boost converter can be unidirectional orbidirectional. For systems shown in FIG. 1(b) and FIG. 1(c), sevenswitches operating at high frequency are required.

Each of FIGS. 2(a) to 2(d) shows a conventional three phase Z sourceinverter (ZSI). Referring to FIGS. 2(a) to 2(d), the ZSI is asingle-stage buck-boost inverter, where the dc supply voltage level canbe pushed up without the need to a separate boost converter. FIG. 2(a)shows the unidirectional version of the ZSI and FIG. 2(b) shows thebidirectional version of the ZSI. The quasi ZSI (qZSI) having a smoothinput current improves performance of the ZSI, thus it is suitable forrenewable energy and energy storage systems. The unidirectional versionand bidirectional version of the qZSI are shown in FIG. 2(c) and 2(d),respectively. Referring to FIGS. 2(a) to 2(d), both of ZSI and qZSI havesix switches operating at high frequency.

FIG. 3(a) shows a unidirectional three phase split source inverter, andFIG. 3(b) shows a bidirectional three phase split source inverter. Thethree phase split source inverter (SSI) is an alternative to the ZSI andqZSI, where the dc supply voltage can be stepped up without the need toa separate boost converter. Referring to FIGS. 3(a) and 3(b), the threephase SSI also has six switches operating at high frequency. Though thebidirectional three phase SSI includes the other three switches, theother three switches do not operate at high frequency but operate at lowfrequency.

FIG. 4(a) shows a generic three phase differential mode inverter (DMI)connection diagram. FIG. 4(b) shows a non-isolated Ćuk three phase DMIand FIG. 4(c) shows an isolated Ćuk three phase DMI. Referring to FIG.4(a), the basic construction of the DMI is three bidirectional buck orbuck-boost DC-DC converters connected in such a way that all of themhave a common ground. Each converter generates an “ac voltage component”with a dc offset, such that the sum is either in the positive ornegative voltage plane. Many DC-DC converters can be used to synthesizethe DMI, like: Sepic, Ćuk, buck-boost . . . etc. Referring to FIGS. 4(b)and 4(c), the least number of switches operating at high frequency for athree-phase DMI is six switches. In FIG. 4(b), a DMI is synthesized withthree bidirectional non-isolated Ćuk converters, while FIG. 4(c) shows aDMI that is synthesized with three bidirectional isolated Ćukconverters.

FIG. 5(a) shows a three phase inverter system using an unfolder circuitaccording to an embodiment of the subject invention. The three phaseinverter can comprise two circuits, each circuit is dedicated togenerate a line-to-line voltage. Each circuit comprises a DC-DCconverter and an unfolder circuit. The DC-DC converter can beunidirectional or bidirectional, and it can be non-isolated or isolated.The main function of this DC-DC converter is to convert the dc inputvoltage to a folded version of a line-to-line voltage. The unfoldercircuit comprises a single phase inverter (H-bridge), that flips thepolarity of the DC-DC converter output voltage every 1/(2×line frequency(50 Hz or 60 Hz)) seconds.

The unfolder circuit comprises four switches operating at the linefrequency (50 Hz or 60 Hz), and the switching loss can be omittedbecause the switches operate at zero voltage. This results in minimalcooling requirement for the unfolder circuit. So, for a three-phaseinverter, two DC-DC converters operating at high frequency and twounfolder circuits operating at the line frequency with negligibleswitching losses are utilized, thereby reducing the total number of highfrequency switches.

The DC-DC converters should be built with power switches that have thecapability to switch at high frequencies with low switching loss.Silicon Carbide (SiC) Mosfets or Gallium Nitride (GaN) HEMTs are usedfor the DC-DC converters' stage. The eight switches of the two unfoldercircuits can be low frequency switches with low drain-to-source onresistance.

Referring to FIG. 5(a), the three phase inverter 100 comprises aconverter circuit 300 including a first DC-DC converter 310 and a secondDC-DC converter 360, and an unfolder circuit 400 including a firstunfolder 410 and a second unfolder 460. That is, the three phaseinverter 100 comprises two DC-DC converters and two unfolders.

The first DC-DC converter 310 is connected to an input source 200through a first converter first input node 311 and a first convertersecond input node 312 such that the first DC-DC converter 310 receivesan input Vin. The first DC-DC converter 310 provides a first converteroutput V₁ through a first converter first output node 316 and a firstconverter second output node 317.

The second DC-DC converter 360 is connected to the input source 200through a second converter first input node 361 and a second convertersecond input node 362 such that the second DC-DC converter 360 receivesthe input Vin. The second DC-DC converter 360 provides a secondconverter output V₂ through a second converter first output node 366 anda second converter second output node 367.

Each of the first unfolder 410 and the second unfolder 460 is a singlephase inverter having an H-bridge structure and includes four switches.The first unfolder 410 includes a first unfolder first input node 411, afirst unfolder second input node 412, a first unfolder first output node416, and a first unfolder second output node 417. The first unfolderfirst input node 411 and the first unfolder second input node 412 areconnected to the first converter first output node 316 and the firstconverter second output node 317, respectively, thereby allowing thefirst unfolder 410 to receive the first converter output V₁ through thefirst unfolder first input node 411 and the first unfolder second inputnode 412.

The first unfolder 410 comprises a first leg 420 including a firstunfolder first switch S₁₁ and a first unfolder second switch S₁₂, and asecond leg 430 including a first unfolder third switch S₁₃ and a firstunfolder fourth switch S₁₄. The first unfolder first switch S₁₁ isconnected to the first unfolder first input node 411, the first unfoldersecond switch S₁₂ is connected to the first unfolder second input node412, and the first unfolder first switch S₁₁ and the first unfoldersecond switch S₁₂ are connected to each other through the first unfolderfirst output node 416. The first unfolder third switch S₁₃ is connectedto the first unfolder first input node 411, the first unfolder fourthswitch S₁₄ is connected to the first unfolder second input node 412, andthe first unfolder third switch S₁₃ and the first unfolder fourth switchS₁₄ are connected to each other through the first unfolder second outputnode 417.

Similar to the first unfolder 410, the second unfolder 460 includes asecond unfolder first input node 461, a second unfolder second inputnode 462, a second unfolder first output node 466, and a second unfoldersecond output node 467. The second unfolder first input node 461 and asecond unfolder second input node 462 are connected to the secondconverter first output node 366 and the second converter second outputnode 367, respectively, thereby allowing the second unfolder 460 toreceive the second converter output V₂ through the second unfolder firstinput node 461 and the second unfolder second input node 462.

The second unfolder 460 comprises a third leg 470 including a secondunfolder first switch S₂₁ and a second unfolder second switch S₂₂, and afourth leg 480 including a second unfolder third switch S₂₃ and a secondunfolder fourth switch S₂₄. The second unfolder first switch S₂₁ isconnected to the second unfolder first input node 461, the secondunfolder second switch S₂₂ is connected to the second unfolder secondinput node 462, and the second unfolder first switch S₂₁ and the secondunfolder second switch S₂₂ are connected to each other through thesecond unfolder first output node 466. The second unfolder third switchS₂₃ is connected to the second unfolder first input node 461, the secondunfolder fourth switch S₂₄ is connected to the second unfolder secondinput node 462, and the second unfolder third switch S₂₃ and the secondunfolder fourth switch S₂₄ are connected to each other through thesecond unfolder second output node 467.

The first unfolder first output node 416 and the second unfolder firstoutput node 466 are connected to each other through an unfolder commonoutput node 456. Thus, the three phase inverter 100 provides a firstphase output V_(ab) between the first unfolder second output node 417and the unfolder common output node 456, a second phase output V_(bc)between the unfolder common output node 456 and the second unfoldersecond output node 467, and a third phase output V_(ca) between thefirst unfolder second output node 417 and the second unfolder secondoutput node 467. A three phase grid 510 or a three phase stand-aloneload 560 is configured to be connected to the first unfolder secondoutput node 417, the unfolder common output node 456, and the secondunfolder second output node 467 as to receive the first phase outputV_(ab), the second phase output V_(bc), and the third phase outputV_(ca). In addition, a first capacitor 610 and a second capacitor 660can be connected to the first DC-DC converter 310 and the second DC-DCconverter 360, respectively.

FIG. 5(b) shows a controller of a three phase inverter system accordingto an embodiment of the subject invention, and FIG. 5(c) shows waveformsof a three phase inverter system according to an embodiment of thesubject invention. A controller 700 generates several signals andcontrols the three phase inverter 100 based on the signals. In thecontroller 700, the reference line-to-line voltages can be calculatedfrom the reference phase voltages. Referring to FIGS. 5(a) to 5(c), thefirst DC-DC converter 310 outputs the first converter output V₁ which isa folded version of V_(ab), while the second DC-DC converter 360 outputsthe second converter output V₂ which is a folded version of V_(bc). Thereference signal V_(ab)* is calculated from the reference signals V_(a)*and V_(b)* through a first differentiator 711, and the reference signalis calculated from V_(b)* and V_(c)* through a second differentiator712. The reference signals V_(ab)* and V_(ab)* are changed to thereference output voltage signals V₁* and V₂* through a first absolutevalue generator 721 and a second absolute value generator 722,respectively. The reference output voltage signals V₁* and V₂* arecalculated in a third differentiator 713 and a fourth differentiator 714with the first converter output V₁ and the second converter output V₂,respectively, and then inputted into a first closed loop controller 741and a second closed loop controller 742, respectively. The closed loopcontrollers 741 and 742, G1(s) and G2(s), track the reference outputvoltage signals V₁* and V₂*, and then provide modulation signals V_(m1)*and V_(m2)* to generate the pulse width modulation signals of the firstDC-DC converter 310 and the second DC-DC converter 360. The controller700 further comprises a first comparator 731 and a second comparator 732to generate the switching signals of the first unfolder 410 and thesecond unfolder 460. The first comparator 731 provides a first signal tothe first unfolder second switch S₁₂ and the first unfolder third switchS₁₃, and a second signal to the first unfolder first switch S₁₁ and thefirst unfolder fourth switch S₁₄. The second signal is generated by afirst inverter 751 from the first signal. The second comparator 732provides a third signal to the second unfolder first switch S₂₁ and thesecond unfolder fourth switch S₂₄, and a fourth signal to the secondunfolder second switch S₂₂ and the second unfolder third switch S₂₃. Thefourth signal is generated by a second inverter 752 from the thirdsignal.

FIG. 6 shows key waveforms of a three phase inverter system according toan embodiment of the subject invention. Referring to FIG. 5(c) and FIG.6, the first phase output V_(ab), the second phase output V_(bc), andthe third phase output V have different phases, and the first converteroutput V₁ and the second converter output V₂ show folded characteristic.In addition, the first to fourth signals to control the unfolderswitches S₁₁ to S₁₄ and S₂₁ to S₂₄ have different phases.

FIG. 7 shows an isolated unidirectional version of a three phaseinverter system for a photovoltaic interface application according to anembodiment of the subject invention. Referring to FIG. 7, a three-phaseisolated unidirectional inverter system with an eight-switch-three-phase(ESTP) unfolder is presented for photovoltaic interfacing application,and comprises an isolated unidirectional Ćuk converter as a DC-DCconverter. Thus, the inverter system comprises only two switchesoperating at high frequency.

FIG. 8 shows a non-isolated bidirectional version of a three phaseinverter system for energy storage interface application according to anembodiment of the subject invention. Referring to FIG. 8, a three-phasenon-isolated bidirectional inverter system with an ESTP unfolder ispresented for energy storage interfacing application, and comprises anon-isolated bidirectional Sepic converter as a DC-DC converter. As aresult, the inverter system comprises four switches operating at highfrequency.

FIG. 9 shows an isolated bidirectional version of a three phase invertersystem for energy storage interface application according to anembodiment of the subject invention. Referring to FIG. 9, a three-phaseisolated bidirectional inverter system with an ESTP unfolder ispresented for energy storage interfacing application, and comprises anisolated bidirectional Ćuk converter as a DC-DC converter. In thisembodiment, the inverter system comprises four switches operating athigh frequency.

Referring to FIGS. 5-9, embodiments of the subject invention may use atmost four switches operating at high frequency, thereby reducing thetotal number of high frequency switching devices. The use of less highfrequency switching devices results in higher efficiency compared tocompetitive inverter systems, and less cooling requirement. In addition,embodiments accomplish the reduction of the cooling system size andweight yields higher power density and higher specific power. Thereduction of the number of the high frequency switches results inreduction in cost because they are expensive, while the low frequencyswitches (for the ESTP unfolder circuit) are inexpensive.

The inverter systems according to embodiments of the subject inventioncan be easily configured to have custom features, such as isolation,boosting the input dc voltage, and smooth input current, and theinverter systems exhibit high quality output voltage without the needfor an output filter.

The subject invention includes, but is not limited to, the followingexemplified embodiments.

Embodiment 1

A three phase inverter, comprising:

a converter circuit providing a first converter output and a secondconverter output;

a first unfolder including a first leg and a second leg that areconnected to the first converter output through a first unfolder firstinput node and a first unfolder second input node; and

a second unfolder including a third leg and a fourth leg that areconnected to the second converter output through a second unfolder firstinput node and a second unfolder second input node,

a first unfolder first output node of the first leg being connected to asecond unfolder first output node of the third leg through an unfoldercommon output node,

the second leg providing a first unfolder second output node,

the fourth leg providing a second unfolder second output node,

each of the first to fourth leg comprising two switches.

Embodiment 2

The three phase inverter according to embodiment 1, the switchesoperating at a line frequency.

Embodiment 3

The three phase inverter according to any of embodiments 1-2, the threephase inverter providing a first phase output between the first unfoldersecond output node and the unfolder common output node, a second phaseoutput between the unfolder common output node and the second unfoldersecond output node, and a third phase output between the first unfoldersecond output node and the second unfolder second output node.

Embodiment 4

The three phase inverter according to any of embodiments 1-3, the firstleg comprising a first unfolder first switch connected between the firstunfolder first input node and the first unfolder first output node and afirst unfolder second switch connected between the first unfolder firstoutput node and the first unfolder second input node; and the second legcomprising a first unfolder third switch connected between the firstunfolder first input node and the first unfolder second output node anda first unfolder fourth switch connected between the first unfoldersecond output node and the first unfolder second input node.

Embodiment 5

The three phase inverter according to any of embodiments 1-4, the thirdleg comprising a second unfolder first switch connected between thesecond unfolder first input node and the second unfolder first outputnode and a second unfolder second switch connected between the secondunfolder first output node and the second unfolder second input node;and the fourth leg comprising a second unfolder third switch connectedbetween the second unfolder first input node and the second unfoldersecond output node and a second unfolder fourth switch connected betweenthe second unfolder second output node and the second unfolder secondinput node.

Embodiment 6

The three phase inverter according to any of embodiments 1-5, theconverter circuit comprising a first DC-DC converter receiving an inputvoltage and providing the first converter output and a second DC-DCconverter receiving the input voltage and providing the second converteroutput.

Embodiment 7

The three phase inverter according to embodiment 6, each of the firstand second DC-DC converters being at least one of an isolatedunidirectional DC-DC converter, a non-isolated bidirectional DC-DCconverter, and an isolated bidirectional DC-DC converter.

Embodiment 8

The three phase inverter according to any of embodiments 6-7, furthercomprising a controller including a first comparator providing a firstsignal and a second signal that control the first unfolder, and a secondcomparator providing a third signal and a fourth signal that control thesecond unfolder.

Embodiment 9

The three phase inverter according to embodiment 8, the controllerproviding the first signal to the first unfolder second switch and thefirst unfolder third switch, and providing the second signal to thefirst unfolder first switch and the first unfolder fourth switch, andthe second signal being generated through a first inverter from thefirst signal.

Embodiment 10

The three phase inverter according to any of embodiments 8-9, thecontroller providing the third signal to the second unfolder firstswitch and the second unfolder fourth switch, and providing the fourthsignal to the second unfolder second switch and the second unfolderthird switch, and the fourth signal being generated through a secondinverter from the third signal.

Embodiment 11

The three phase inverter according to any of embodiments 6-10, thecontroller providing a first modulation signal to the first DC-DCconverter and a second modulation signal to the second DC-DC converter.

Embodiment 12

A three phase inverter, comprising:

a first converter and a second converter connected to an input source inparallel, respectively;

a first single phase inverter connected to the first converter through afirst inverter first input node and a first inverter second input nodeand providing a first inverter first output node and a first invertersecond output node;

a second single phase inverter connected to the second converter througha second inverter first input node and a second inverter second inputnode and providing a second inverter first output node and a secondinverter second output node; and

a common output node connected to the first inverter first output nodeand the second inverter first output node.

Embodiment 13

The three phase inverter according to embodiment 12, the first singlephase inverter comprising a first inverter first switch connectedbetween the first inverter first input node and the first inverter firstoutput node, a first inverter second switch connected between the firstinverter first output node and the first inverter second input node, afirst inverter third switch connected between the first inverter firstinput node and the first inverter second output node, and a firstinverter fourth switch connected between the first inverter secondoutput node and the first inverter second input node.

Embodiment 14

The three phase inverter according to any of embodiments 12-13, thesecond single phase inverter comprising a second inverter first switchconnected between the second inverter first input node and the secondinverter first output node, a second inverter second switch connectedbetween the second inverter first output node and the second invertersecond input node, a second inverter third switch connected between thesecond inverter first input node and the second inverter second outputnode, and a second inverter fourth switch connected between the secondinverter second output node and the second inverter second input node.

Embodiment 15

The three phase inverter according to embodiment 14, the first inverterfirst switch to the first inverter fourth switch and the second inverterfirst switch to the second inverter fourth switch being operated at aline frequency.

Embodiment 16

The three phase inverter according to any of embodiments 12-15, furthercomprising a controller providing a first signal and a second signalthat control the first single phase inverter, and providing a thirdsignal and a fourth signal that control the second single phaseinverter.

Embodiment 17

The three phase inverter according to embodiment 16, the first signalbeing provided to the first inverter second switch and the firstinverter third switch, and second signal being provided to the firstinverter first switch and the first inverter fourth switch.

Embodiment 18

The three phase inverter according to any of embodiments 16-17, thethird signal being provided to the second inverter first switch and thesecond inverter fourth switch, and the fourth signal being provided tothe second inverter second switch and the second inverter third switch.

Embodiment 19

The three phase inverter according to any of embodiments 16-18, thesecond signal being inverted from the first signal, and the fourthsignal being inverted from the third signal.

Embodiment 20

A three phase inverter, comprising:

a first DC-DC converter connected to an input source;

a second DC-DC converter connected to the input source;

a first inverter first switch connected to the first DC-DC converterthrough a first inverter first input node;

a first inverter second switch connected to the first DC-DC converterthrough a first inverter second input node and connected to the firstinverter first switch through a first inverter first output node;

a first inverter third switch connected to the first DC-DC converterthrough the first inverter first input node;

a first inverter fourth switch connected to the first DC-DC converterthrough the first inverter second input node and connected to the firstinverter third switch through a first inverter second output node;

a second inverter first switch connected to the second DC-DC converterthrough a second inverter first input node;

a second inverter second switch connected to the second DC-DC converterthrough a second inverter second input node and connected to the secondinverter first switch through a second inverter first output node;

a second inverter third switch connected to the second DC-DC converterthrough the second inverter first input node;

a second inverter fourth switch connected to the second DC-DC converterthrough the second inverter second input node and connected to thesecond inverter third switch through a second inverter second outputnode;

a common output node connected to the first inverter first output nodeand the second inverter first output node; and

a controller providing a first signal to the first inverter secondswitch and the first inverter third switch, a second signal to the firstinverter first switch and the first inverter fourth switch, a thirdsignal to the second inverter first switch and the second inverterfourth switch, and a fourth signal to the second inverter second switchand the second inverter third switch.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication.

All patents, patent applications, provisional applications, andpublications referred to or cited herein are incorporated by referencein their entirety, including all figures and tables, to the extent theyare not inconsistent with the explicit teachings of this specification.

1. A three phase inverter, comprising: a converter circuit generating afirst converter output and a second converter output; a first unfolderincluding a first leg and a second leg that are connected to the firstconverter output via a first unfolder first input node and a firstunfolder second input node; and a second unfolder including a third legand a fourth leg that are connected to the second converter output via asecond unfolder first input node and a second unfolder second inputnode, the first leg including a first unfolder first output node, thesecond leg including a first unfolder second output node, the third legincluding a second unfolder first output node, the fourth leg includinga second unfolder second output node, each of the first to fourth legcomprising two switches, an unfolder common output node being connectedto the first unfolder first output node and the second unfolder firstoutput node, and the three phase inverter generating a first phaseoutput between the first unfolder second output node and the unfoldercommon output node, a second phase output between the unfolder commonoutput node and the second unfolder second output node, and a thirdphase output between the first unfolder second output node and thesecond unfolder second output node.
 2. The three phase inverteraccording to claim 1, the switches operating at a line frequency. 3.(canceled)
 4. The three phase inverter according to claim 1, the firstleg comprising a first unfolder first switch connected between the firstunfolder first input node and the first unfolder first output node and afirst unfolder second switch connected between the first unfolder firstoutput node and the first unfolder second input node; and the second legcomprising a first unfolder third switch connected between the firstunfolder first input node and the first unfolder second output node anda first unfolder fourth switch connected between the first unfoldersecond output node and the first unfolder second input node.
 5. Thethree phase inverter according to claim 4, the third leg comprising asecond unfolder first switch connected between the second unfolder firstinput node and the second unfolder first output node and a secondunfolder second switch connected between the second unfolder firstoutput node and the second unfolder second input node; and the fourthleg comprising a second unfolder third switch connected between thesecond unfolder first input node and the second unfolder second outputnode and a second unfolder fourth switch connected between the secondunfolder second output node and the second unfolder second input node.6. The three phase inverter according to claim 5, the converter circuitcomprising a first DC-DC converter receiving an input voltage andgenerating the first converter output and a second DC-DC converterreceiving the input voltage and generating the second converter output.7. The three phase inverter according to claim 6, each of the first andsecond DC-DC converters being at least one of an isolated unidirectionalDC-DC converter, a non-isolated bidirectional DC-DC converter, and anisolated bidirectional DC-DC converter.
 8. The three phase inverteraccording to claim 6, further comprising a controller including a firstcomparator generating a first signal and a second signal that controlthe first unfolder, and a second comparator generating a third signaland a fourth signal that control the second unfolder.
 9. The three phaseinverter according to claim 8, the controller supplying the first signalto the first unfolder second switch and the first unfolder third switch,and supplying the second signal to the first unfolder first switch andthe first unfolder fourth switch, and a first inverter receiving thefirst signal and generating the second signal.
 10. The three phaseinverter according to claim 9, the controller supplying the third signalto the second unfolder first switch and the second unfolder fourthswitch, and supplying the fourth signal to the second unfolder secondswitch and the second unfolder third switch, and a second inverterreceiving the third signal and generating the fourth signal.
 11. Thethree phase inverter according to claim 10, the controller supplying afirst modulation signal to the first DC-DC converter and a secondmodulation signal to the second DC-DC converter.
 12. A three phaseinverter, comprising: a first converter and a second converter connectedto an input source in parallel, respectively; a first single phaseinverter connected to the first converter via a first input node and asecond input node and including a first output node and a second outputnode; a second single phase inverter connected to the second convertervia a third input node and a fourth input node and including a thirdoutput node and a fourth output node; and a common output node connectedto the first output node and the third output node, the three phaseinverter generating a first phase output between the second output nodeand the common output node, a second phase output between the commonoutput node and the fourth output node, and a third phase output betweenthe second output node and the fourth output node.
 13. The three phaseinverter according to claim 12, the first single phase invertercomprising a first inverter first switch connected between the firstinput node and the first output node, a first inverter second switchconnected between the first output node and the second input node, afirst inverter third switch connected between the first input node andthe second output node, and a first inverter fourth switch connectedbetween the second output node and the second input node.
 14. The threephase inverter according to claim 13, the second single phase invertercomprising a second inverter first switch connected between the thirdinput node and the third output node, a second inverter second switchconnected between the third output node and the fourth input node, asecond inverter third switch connected between the third input node andthe fourth output node, and a second inverter fourth switch connectedbetween the fourth output node and the fourth input node.
 15. The threephase inverter according to claim 14, the first inverter first switch tothe first inverter fourth switch and the second inverter first switch tothe second inverter fourth switch being operated at a line frequency.16. The three phase inverter according to claim 14, further comprising acontroller generating a first signal and a second signal that controlthe first single phase inverter, and generating a third signal and afourth signal that control the second single phase inverter.
 17. Thethree phase inverter according to claim 16, the first signal beingsupplied to the first inverter second switch and the first inverterthird switch, and second signal being supplied to the first inverterfirst switch and the first inverter fourth switch.
 18. The three phaseinverter according to claim 17, the third signal being supplied to thesecond inverter first switch and the second inverter fourth switch, andthe fourth signal being supplied to the second inverter second switchand the second inverter third switch.
 19. The three phase inverteraccording to claim 18, the second signal being inverted from the firstsignal, and the fourth signal being inverted from the third signal. 20.A three phase inverter, comprising: a first DC-DC converter connected toan input source; a second DC-DC converter connected to the input source;a first inverter first switch connected to the first DC-DC converter viaa first inverter first input node; a first inverter second switchconnected to the first DC-DC converter via a first inverter second inputnode and connected to the first inverter first switch via a firstinverter first output node; a first inverter third switch connected tothe first DC-DC converter via the first inverter first input node; afirst inverter fourth switch connected to the first DC-DC converter viathe first inverter second input node and connected to the first inverterthird switch via a first inverter second output node; a second inverterfirst switch connected to the second DC-DC converter via a secondinverter first input node; a second inverter second switch connected tothe second DC-DC converter via a second inverter second input node andconnected to the second inverter first switch via a second inverterfirst output node; a second inverter third switch connected to thesecond DC-DC converter via the second inverter first input node; asecond inverter fourth switch connected to the second DC-DC convertervia the second inverter second input node and connected to the secondinverter third switch via a second inverter second output node; a commonoutput node connected to the first inverter first output node and thesecond inverter first output node; and a controller supplying a firstsignal to the first inverter second switch and the first inverter thirdswitch, a second signal to the first inverter first switch and the firstinverter fourth switch, a third signal to the second inverter firstswitch and the second inverter fourth switch, and a fourth signal to thesecond inverter second switch and the second inverter third switch, thethree phase inverter generating a first phase output between the firstinverter second output node and the common output node, a second phaseoutput between the common output node and the second inverter secondoutput node, and a third phase output between the first inverter secondoutput node and the second inverter second output node.